1. Field of the Invention
The present invention concerns friction materials and their use in braking devices and, more particularly, a multimaterial disk for high-energy braking.
Optimizing the friction materials used in the braking systems of high-speed trains is proving increasingly difficult because of the huge amounts of energy to be dissipated. In this context of particularly active research, conventional solutions like the brake disks of the French "Atlantic" (TGV-A) high-speed train in conjunction with sintered iron-copper linings, for example, are currently reaching their limits and the maximum speed of 300 kph for passenger trains is due in part to our current inability to design braking systems of adequate performance. At very high speeds only friction brakes are capable of stopping a train.
2. Description of the Prior Art
Prior art braking devices employ, for example, a caliper device requiring the use of brake disks which are usually mounted on the axle.
In these devices, brake pads bite into the brake disks from both sides.
Optimizing the friction materials constituting certain parts of the braking devices of high-speed trains or other heavy vehicles is increasingly difficult because of the huge amounts of energy to be dissipated. This energy is proportional to the square of the speed and the mass in movement.
The magnitude of the energy to be dissipated is also high in the case of frequent braking, as in trains on underground railways, for example.
Prior art braking devices using TGV-A brake disks and sintered iron-copper facings, for example, are currently reaching their limits.
For example, each disk of the prior art braking devices on high-speed trains dissipates on the order of 14 MJ on braking from a speed on the order of 300 kph. The maximal energy that can be dissipated is on the order of 19 MJ.
The current target in the braking art is for each disk to dissipate around 25 MJ on braking from a speed on the order of 400 kph.
Ceramic brakes constitute an interesting alternative. Ceramic materials generally have high specific heat and mechanical strength at high temperature, good friction properties and in some cases good resistance to thermal shock. However, solid ceramics are very fragile with the result that it is not feasible to envisage a reliable brake disk of unitary construction. On the other hand, the use of a ceramic facing or insert seems more realistic from the reliability and cost points of view. Feasible options include multimaterial applications such as ceramic facings built up on the disk by spraying and disks comprising an assembly of various flanges with sintered ceramic linings. This technology, initially developed for trains (high-speed trains, trains which have to stop frequently, etc), are susceptible to further development for other types of vehicles (heavy goods vehicles, etc.) likely to generate high levels of energy to be dissipated in a braking configuration or for any other braking application in which a stable coefficient of friction and little wear of the friction materials would be beneficial.
Friction braking raises the problem of dissipating the heat. The heat generated by braking near the wheel hubs must not be communicated to the wheel bearings which are sensitive to heat. Very intense heating may cause the wheel to lock up on its axle. Ceramic and cermet (ceramic-metal) facings or inserts are intended to limit the propagation of heat into the axle, to improve the wear resistance of the disk/lining combination and its resistance to thermal fatigue, and to offer stable friction characteristics at high temperature; this has led to the development of a new type of lining.
Document GB-A-2 268 511 describes a brake disk including at least one ceramic or cermet facing and a nickel-based thermal barrier.
An object of the present invention is to propose a multimaterial disk for high-energy braking in high-speed trains using ceramic materials. The invention concerns only the disk which in all embodiments operates in conjunction with a ceramic lining as described in the prior art.
Another object of the present invention is to propose friction materials suitable for use in a brake disk and capable of limiting the propagation of heat into the axle.
Another object of the present invention is to propose friction materials suitable for use in a brake disk and capable of improving the wear resistance of the disk/lining combination.
Another object of the present invention is to propose friction materials suitable for use in a brake disk and capable of improving the resistance of the disk to thermal fatigue in combination with stable friction characteristics at high temperature.
In this document the term "cermet" is an abbreviation of "ceramic/metal" and refers to a ceramic-metal composite material.